Crankcase structure of internal combustion engine

Abstract

A bearing holding portion supporting a crankshaft in a manner to rotate freely via a bearing and fittingly holding the bearing is formed in a crankcase. A second cross section passing through a crankshaft axis line and along an orthogonal direction to a cylinder axis line is set to have a larger cross-sectional area than a first cross section passing through the crankshaft axis line and along a direction of the cylinder axis line in the bearing holding portion.

What is claimed is:  
     
       1. A crankcase structure of an internal combustion engine, the crankcase structure comprising:
 a bearing holding portion formed in a crankcase, the bearing holding portion supporting a crankshaft in a manner to rotate freely via a bearing and fittingly holding the bearing, 
 wherein the bearing holding portion is formed integrally with the crankcase and continuously formed in a crushed round shape along a whole outer periphery of the bearing so as to surround the bearing, 
 wherein a second cross section passing through a crankshaft axis line and along an orthogonal direction to a cylinder axis line is set to have a larger cross-sectional area than a cross-sectional area of a first cross section passing through the crankshaft axis line and along a direction of the cylinder axis line in said bearing holding portion, 
 wherein a length along the orthogonal direction to the cylinder axis line of the second cross section is set larger than a length along the direction of the cylinder axis line of the first cross section, and 
 wherein cross sections located on one side and another other side across the crankshaft axis line in the second cross section are each set larger in cross-sectional area than either larger one of cross sections located on one side and another side across the crankshaft axis line in the first cross section. 
 
     
     
       2. The crankcase structure of the internal combustion engine according to  claim 1 ,
 wherein a pair of said bearing holding portions is separately disposed in a direction of the crankshaft axis line across the cylinder axis line, and 
 wherein a cross-sectional area of the second cross section in at least either one of said bearing holding portions is set larger than that of the first cross section. 
 
     
     
       3. The crankcase structure of the internal combustion engine according to  claim 1 , comprising:
 a plurality of boss portions to which cylinder head bolts are screwed; and 
 a first rib formed to connect said boss portion and said bearing holding portion, 
 wherein a mutual interval of said first ribs is wider than said bearing holding portion. 
 
     
     
       4. The crankcase structure of the internal combustion engine according to  claim 3 , comprising:
 a second rib formed to connect each vicinity of lower end parts of said boss portions. 
 
     
     
       5. The crankcase structure of the internal combustion engine according to  claim 1 ,
 wherein the crankcase is configured to be right and left two-split in relation to the cylinder axis line, each split portion having said bearing holding portion, and the bearing holding portions are each formed integrally with the crankcase and continuously formed in a crushed round shape along a whole outer periphery of the bearing so as to surround the bearing.